Method of treating bottom-hole formation zone

ABSTRACT

The invention relates to the methods of treating a bottom-hole formation zone to increase in well productivity and rocks permeability. According to this method a pulse generator should be tripped in a well and the formation pulse treatment should be conducted by generating negative pressure pulses of amplitude higher than the tensile formation strength. The method provides the high fissuring rate by breaking formation fluid-bearing permeable rocks around a wellbore.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 11/762,392, filed Jun. 13, 2007.

FIELD OF THE INVENTION

The subject disclosure relates to oil and gas well production and can be used to treat a bottom-hole formation zone to increase well productivity and rock permeability.

BACKGROUND OF THE INVENTION

At present, various methods of treating a bottom-hole formation zone are directed to the increase in oil recovery coefficient. These are reactant treatments of the producing formations involving the injection of different processing media based on organic and non-organic matters to a well, pulse methods combined with mechanical; thermal and chemical effect, and hydraulic fracturing of the formation, being a better-known well stimulation of hydrocarbons through increase in permeability of the bottom-hole zone of the producing formation due to fissuring.

The methods of treating a bottom-hole zone involving pressure pulses are based on elastic wave/pressure wave excitation in rock formation. The pressure wave effect was proposed more than 40 years ago as an alternative procedure resulting in higher efficiency of the standard methods. This method has not found a wide application yet despite some beneficial results in practice (e.g. flow rate increase and/or oil recovery coefficient). The central problem is the lack of reliable field data and theoretical reasoning too. Particularly, it is impossible to predict or stimulate what is the effect (positive or negative) of pressure pulses on production. Nevertheless, some equipment has been developed, among them surface vibrators and downhole tool (pressure pulse excitation tool, sparkers, magnetostrictive and piezoceramic sources), which results a wide range of pulse frequency.

A most close analog to a method applied is a method of treating a bottom-hole zone involving the trip of a pulse generator in a well followed by the formation pulse treatment specified in patent RU 2105874, 1998.

SUMMARY OF THE INVENTION

The present invention provides a method of treating a bottom-hole zone that provides high fissuring rate by breaking formation fluid-bearing permeable rocks around a wellbore. This method increases the rock permeability through the generation of formation microfractures or the regeneration of earlier fissures; and combined with the hydraulic fracturing provided that fractures propagate and reach the surface of the hydraulic fracturing fissures the pressure pulses form rock lumps that do away with the fissure surface and become proppants themselves.

In the present invention a provision is made for the method of treating a bottom-hole zone involving the trip of a pulse generator in a well followed by the formation pulse treatment to generate the negative pressure pulses of amplitude higher than tensile formation strength and with duration less than the character time of diffusion of fluid in reservoir. If the characteristic diffusion time of fluid in a reservoir is T, then the duration of a pulse can be set as 1/100 of T which is sufficient. The diffusion time for fluid in a reservoir is estimated as T=R̂2/D, where R is the radius of a wellbore and D is the coefficient of diffusion of a fluid. For example, if the character diffusion time is T=1 sec, then the duration of a pulse can be set at 0.01 sec.

In case of hydraulic formation fracturing, pressure pulses are fed as a breaking fissure grows. Moreover, prior to pulse action the pressure is built in a bottom-hole well zone higher than pore pressure in a far-field zone for the formation; or in case of hydraulic fracturing the pressure is built in the created fracture higher than principle maximum stress in the far-field zone for the formation.

DETAILED DESCRIPTION OF THE INVENTION

The invention is carried out as follows. A pulse generator should be tripped in a well and negative pressure pulses be generated around oil-bearing formation of amplitude higher than the tensile formation strength. A short and power pulse with magnitude of several MPa and duration less than the character time of diffusion of fluid in reservoir can initiate fissuring near a wellbore and in a created fracture (in case of hydraulic fracturing). Each next negative pressure pulse should make formation fissures grow. In case of hydraulic formation fracturing, pressure pulses can be fed as a breaking fissure grows. To create ruptures prior to pulse action the pressure is built in a bottom-hole well zone higher than pore pressure in a far-field zone for the formation; or in case of hydraulic fracturing the pressure is built in the created fracture higher than the principle maximum stress in the far-field zone for the formation.

As an example let us consider an axisymmentric well of radius R being drilled straight, and the hydraulic fracturing (straight and vertical) of L long is in a permeable rock formation. The well cavity and the hydraulic fracturing are filled with fluid at a certain pressure P_(w). For a well P_(w)>p₀, for hydraulic fracturing P_(w)>−σ₁ ^((f)), where, p₀ is the pore pressure in the far-field zone (e.g. 5 MPa), and σ₁ ^((f)) is the principle maximum stress in the far-field zone (e.g. 8 MPa) (it is taken that the tensile stress is positive). The pressure P_(w) has been applied for the set time to build up excessive pressure in the formation (i.e. fluid diffusion process). Elastic motion in the fluid-bearing pore medium is described by the following equations for a medium displacement vector u and a relative fluid displacement vector w:

$\begin{matrix} {{{{\rho \overset{¨}{\overset{\rightarrow}{u}}} + {\rho_{f}\overset{¨}{\overset{\rightarrow}{w}}}} = {{G\; \Delta \overset{\rightarrow}{u}} + {\overset{\rightarrow}{\nabla}\left\lbrack {{\left( {K + {\frac{1}{3}G} + {\alpha^{2}M}} \right)\left( {\overset{\rightarrow}{\nabla}\overset{\rightarrow}{u}} \right)} + {\alpha \; {M\left( {\overset{\rightarrow}{\nabla}\overset{\rightarrow}{w}} \right)}}} \right\rbrack}}},} & \left( {1a} \right) \\ {{{\rho_{f}\overset{¨}{\overset{\rightarrow}{u}}} + {\frac{T_{\varphi}}{\varphi}\rho_{f}\overset{¨}{\overset{\rightarrow}{w}}} + {\frac{\mu}{\kappa}\overset{.}{\overset{\rightarrow}{w}}}} = {{\overset{\rightarrow}{\nabla}\left\lbrack {{\alpha \; {M\left( {\overset{\rightarrow}{\nabla}\overset{\rightarrow}{u}} \right)}} + {M\left( {\overset{\rightarrow}{\nabla}\overset{\rightarrow}{w}} \right)}} \right\rbrack}.}} & \left( {1b} \right) \end{matrix}$

Where, p is the total mass density of the saturated rock, p_(f) is the pore fluid mass density, G is the shear modulus, K is the bulk modulus under drainage, M is the BioH modulus, α is the elastic pore medium coefficient, φ is the porosity, T φ is the rock pore tortuosity coefficient, μ is the fluid viscosity, k is the rock permeability, and a point is the time derivative. Stress components and the pore pressure are in the form of the first space derivative ū and w:

$\begin{matrix} {{\sigma_{ij} = {{2G\; e_{ij}} + {\delta_{ij}\left( {{\left( {K - {\frac{2}{3}G} + {\alpha^{2}M}} \right)e} - {\alpha \; M\; \zeta}} \right)}}},} & \left( {2a} \right) \\ {p = {{{- \alpha}\; M\; e} + {M\; {\zeta.}}}} & \left( {2b} \right) \end{matrix}$

Where,

${e_{ij} = {{1/2}\left( {{{\partial u_{i}}/{\partial x_{j}}} + {{\partial u_{j}}/{\partial x_{i}}}} \right)}},{e = {\sum\limits_{i}{{\partial u_{i}}/{\partial x_{i}}}}},\mspace{14mu} {\zeta = {- {\sum\limits_{i}{{\partial w_{i}}/{{\partial x_{i}}.}}}}}$

At the interface between the well fluid and the porous reservoir the following conditions are satisfied:

σ_(nn) =−P, σ _(nτ)=0, p=P  (3)

Where, the left-hand side of the equations has normal stress, shear stress and pore pressure, respectively, and P=P_(w)+P(t) is the total pressure of the well fluid. Solving a problem (1) of the boundary conditions (3) for the wellbore and hydraulic fracturing gives the space stress and pore pressure distribution. The use of the below known criteria of the tensile failures and the failures according to a Mohr-Coulomb law is the possibility of estimating the tensile rock failure and the failure by shear fractures:

$\begin{matrix} {{{g_{TC} \equiv \sigma_{1}^{eff}} = {{\sigma_{1} + p} > T_{0}}},} & \left( {4a} \right) \\ {{g_{MC} \equiv {{\sigma_{1}t\; {g^{2}\left( {\frac{\pi}{4} + \frac{\phi}{2}} \right)}} - \sigma_{3}} \geq \sigma_{c}},} & \left( {4b} \right) \end{matrix}$

Where, g_(TC) and g_(MC) are the function of fissure flow for ruptures and shear fractures, respectively, being analyzed to predict rock fracturing; T₀ and σ_(c) are the tensile strength and the crushing strength of the rock, respectively.

Dynamic pulses P(t) applied are of negative amplitude, for example, P(t)=−P-pulse exp(−t²/T²pulse), where, P-pulse is the amplitude, and T-pulse is the pulse period.

Should the tensile formation strength T₀ is 1 MPa, the amplitude P-pulse is rather powerful, e.g. 5 MPa, and the T-pulse duration for rock permeability k equal to 10⁻³ is rather short, e.g. 0.01 s; ruptures and shear fractures occurring around wellbore and created fractures.

A fissure propagation direction can be predicted by the nature of the fissures themselves, i.e. ruptures or shear fractures. With pressure reduced, a maximum tensile component is radial relative to a wellbore wall and normal relative to a fissure direction at the surface of the fracturing. Therefore, ruptures propagate in parallel to the wellbore boundary or a created fracture. Shear fractures, if any, are inclined at an angle ψ_(c)=π/4−φ/2 to the direction of principle minimum stress, where, φ is the rock friction angle. 

1. A method of treating a bottom-hole formation zone, the method comprising the steps of: tripping a pulse generator in a well; pressurizing a bottom-hole zone higher than the pore pressure or the principle maximum stress in a far-field zone of the formation; generating at least one negative pressure pulse with magnitude higher than ultimate tensile formation strength and duration less than a character time of diffusion of a fluid in a reservoir.
 2. The method of claim 1 wherein the at least one negative pressure pulse is applied in a process of hydraulic fracturing during a fracture propagation.
 3. The method of claim 1 wherein the at least one negative pressure pulse has magnitude of 5 MPa.
 4. The method of claim 1 wherein a duration of the at least one negative pressure pulse is 0.01 s. 